Vitamin E (d-alpha-tocopherol, 1) is an important nutritional supplement in humans and animals. Compound 1 is obtained commercially by isolation from a variety of plant oils, or semisynthetically by ring methylation of the related naturally occurring d-gamma-tocopherol 2. A more important source of vitamin E is total synthesis, which provides d,l-alpha-tocopherol 3. Although a mixture of isomers, 3 provides much of the biological activity of 1 and is widely used due to its lower cost and greater availability. For a general discussion of vitamin E, see L. Machlin, ed., "Vitamin E: A Comprehensive Treatise", Marcel Dekker, N.Y., 1980. ##STR1##
d,l-alpha-Tocopherol 3 is obtained by reacting trimethylhydroquinone 4 with either phytol 5 or isophytol 6 in the presence of an acid catalyst, often a Lewis acid such as zinc chloride. This technology was reviewed by S. Kasparek in L. Machlin, ed., Vitamin E: A Comprehensive Treatise, chapter 2, pp. 8-65, Marcel Dekker, N.Y., 1965. References 140-166 of this chapter provide the primary references to detailed methods of preparing compound 3. ##STR2##
The isophytol 6 or phytol 5 required for the preparation of 3 are obtained through multistep synthesis. Starting materials typically include acetone (see Kasparek, in Machlin, Vitamin E: A Comprehensive Treatise, pp. 44-45, Dekker, N.Y. 1980, and references cited therein) and cyclic isoprene trimer (Pond et al., U.S. Pat. No. 3,917,710 and U.S. Pat. No. 3,862,988 (1975).
In addition, gamma-tocopherol was first made by total synthesis by Jacob, Steiger, Todd and Wilcox (J. Chem. Soc. 1939, 542). These workers produced the vitamin by reacting the monobenzoate ester of 2,3-dimethylhydroquinone with phytyl bromide in the presence of zinc chloride, followed by removal of the benzoate to give a low yield (22%) of gamma-tocopherol. Published syntheses of the tocopherols and tocotrienols were reviewed by S. Kasparek in L. Machlin, "Vitamin E--A Comprehensive Treatise", Dekker, N.Y., 1980; however, no further methods of preparing gamma-tocopherol have been recorded.
Despite the various known methods for preparing or isolating members of the vitamin E family of compounds, there remains a need for improved and more efficient methods of production.